2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
4 \section[WorkWrap]{Worker/wrapper-generating back-end of strictness analyser}
7 module WorkWrap ( wwTopBinds, mkWrapper ) where
9 #include "HsVersions.h"
12 import CoreUnfold ( certainlyWillInline )
13 import CoreUtils ( exprType, exprIsHNF )
14 import CoreArity ( exprArity )
16 import Id ( Id, idType, isOneShotLambda,
17 setIdNewStrictness, mkWorkerId,
18 setIdWorkerInfo, setInlinePragma,
20 import MkId ( lazyIdKey, lazyIdUnfolding )
23 import NewDemand ( Demand(..), StrictSig(..), DmdType(..), DmdResult(..),
24 Demands(..), mkTopDmdType, isBotRes, returnsCPR, topSig, isAbsent
27 import Unique ( hasKey )
28 import BasicTypes ( RecFlag(..), isNonRec, isNeverActive )
29 import VarEnv ( isEmptyVarEnv )
30 import Maybes ( orElse )
32 import Util ( lengthIs, notNull )
37 We take Core bindings whose binders have:
41 \item Strictness attached (by the front-end of the strictness
44 \item Constructed Product Result information attached by the CPR
49 and we return some ``plain'' bindings which have been
50 worker/wrapper-ified, meaning:
54 \item Functions have been split into workers and wrappers where
55 appropriate. If a function has both strictness and CPR properties
56 then only one worker/wrapper doing both transformations is produced;
58 \item Binders' @IdInfos@ have been updated to reflect the existence of
59 these workers/wrappers (this is where we get STRICTNESS and CPR pragma
60 info for exported values).
64 wwTopBinds :: UniqSupply -> [CoreBind] -> [CoreBind]
66 wwTopBinds us top_binds
68 top_binds' <- mapM wwBind top_binds
69 return (concat top_binds')
72 %************************************************************************
74 \subsection[wwBind-wwExpr]{@wwBind@ and @wwExpr@}
76 %************************************************************************
78 @wwBind@ works on a binding, trying each \tr{(binder, expr)} pair in
79 turn. Non-recursive case first, then recursive...
83 -> UniqSM [CoreBind] -- returns a WwBinding intermediate form;
84 -- the caller will convert to Expr/Binding,
87 wwBind (NonRec binder rhs) = do
89 new_pairs <- tryWW NonRecursive binder new_rhs
90 return [NonRec b e | (b,e) <- new_pairs]
91 -- Generated bindings must be non-recursive
92 -- because the original binding was.
95 = return . Rec <$> concatMapM do_one pairs
97 do_one (binder, rhs) = do new_rhs <- wwExpr rhs
98 tryWW Recursive binder new_rhs
101 @wwExpr@ basically just walks the tree, looking for appropriate
102 annotations that can be used. Remember it is @wwBind@ that does the
103 matching by looking for strict arguments of the correct type.
104 @wwExpr@ is a version that just returns the ``Plain'' Tree.
107 wwExpr :: CoreExpr -> UniqSM CoreExpr
109 wwExpr e@(Type _) = return e
110 wwExpr e@(Lit _) = return e
111 wwExpr e@(Note InlineMe _) = return e
112 -- Don't w/w inside InlineMe's
115 | v `hasKey` lazyIdKey = return lazyIdUnfolding
116 | otherwise = return e
117 -- HACK alert: Inline 'lazy' after strictness analysis
118 -- (but not inside InlineMe's)
120 wwExpr (Lam binder expr)
121 = Lam binder <$> wwExpr expr
124 = App <$> wwExpr f <*> wwExpr a
126 wwExpr (Note note expr)
127 = Note note <$> wwExpr expr
129 wwExpr (Cast expr co) = do
130 new_expr <- wwExpr expr
131 return (Cast new_expr co)
133 wwExpr (Let bind expr)
134 = mkLets <$> wwBind bind <*> wwExpr expr
136 wwExpr (Case expr binder ty alts) = do
137 new_expr <- wwExpr expr
138 new_alts <- mapM ww_alt alts
139 return (Case new_expr binder ty new_alts)
141 ww_alt (con, binders, rhs) = do
142 new_rhs <- wwExpr rhs
143 return (con, binders, new_rhs)
146 %************************************************************************
148 \subsection[tryWW]{@tryWW@: attempt a worker/wrapper pair}
150 %************************************************************************
152 @tryWW@ just accumulates arguments, converts strictness info from the
153 front-end into the proper form, then calls @mkWwBodies@ to do
156 We have to BE CAREFUL that we don't worker-wrapperize an Id that has
157 already been w-w'd! (You can end up with several liked-named Ids
158 bouncing around at the same time---absolute mischief.) So the
159 criterion we use is: if an Id already has an unfolding (for whatever
160 reason), then we don't w-w it.
162 The only reason this is monadised is for the unique supply.
164 Note [Don't w/w inline things]
165 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
166 It's very important to refrain from w/w-ing an INLINE function
167 If we do so by mistake we transform
168 f = __inline (\x -> E)
170 f = __inline (\x -> case x of (a,b) -> fw E)
171 fw = \ab -> (__inline (\x -> E)) (a,b)
172 and the original __inline now vanishes, so E is no longer
173 inside its __inline wrapper. Death! Disaster!
175 Furthermore, if the programmer has marked something as INLINE,
176 we may lose by w/w'ing it.
178 If the strictness analyser is run twice, this test also prevents
179 wrappers (which are INLINEd) from being re-done.
181 Notice that we refrain from w/w'ing an INLINE function even if it is
182 in a recursive group. It might not be the loop breaker. (We could
183 test for loop-breaker-hood, but I'm not sure that ever matters.)
187 -> Id -- The fn binder
188 -> CoreExpr -- The bound rhs; its innards
190 -> UniqSM [(Id, CoreExpr)] -- either *one* or *two* pairs;
191 -- if one, then no worker (only
192 -- the orig "wrapper" lives on);
193 -- if two, then a worker and a
195 tryWW is_rec fn_id rhs
196 | -- isNonRec is_rec && -- Now omitted: see Note [Don't w/w inline things]
197 certainlyWillInline unfolding
199 || isNeverActive inline_prag
200 -- No point in worker/wrappering if the thing is never inlined!
201 -- Because the no-inline prag will prevent the wrapper ever
202 -- being inlined at a call site.
203 = return [ (new_fn_id, rhs) ]
205 | is_thunk && worthSplittingThunk maybe_fn_dmd res_info
206 = ASSERT2( isNonRec is_rec, ppr new_fn_id ) -- The thunk must be non-recursive
207 splitThunk new_fn_id rhs
209 | is_fun && worthSplittingFun wrap_dmds res_info
210 = splitFun new_fn_id fn_info wrap_dmds res_info inline_prag rhs
213 = return [ (new_fn_id, rhs) ]
216 fn_info = idInfo fn_id
217 maybe_fn_dmd = newDemandInfo fn_info
218 unfolding = unfoldingInfo fn_info
219 inline_prag = inlinePragInfo fn_info
221 -- In practice it always will have a strictness
222 -- signature, even if it's a uninformative one
223 strict_sig = newStrictnessInfo fn_info `orElse` topSig
224 StrictSig (DmdType env wrap_dmds res_info) = strict_sig
226 -- new_fn_id has the DmdEnv zapped.
227 -- (a) it is never used again
228 -- (b) it wastes space
229 -- (c) it becomes incorrect as things are cloned, because
230 -- we don't push the substitution into it
231 new_fn_id | isEmptyVarEnv env = fn_id
232 | otherwise = fn_id `setIdNewStrictness`
233 StrictSig (mkTopDmdType wrap_dmds res_info)
235 is_fun = notNull wrap_dmds
236 is_thunk = not is_fun && not (exprIsHNF rhs)
238 ---------------------
239 splitFun :: Id -> IdInfo -> [Demand] -> DmdResult -> InlinePragInfo -> Expr Var
240 -> UniqSM [(Id, CoreExpr)]
241 splitFun fn_id fn_info wrap_dmds res_info inline_prag rhs
242 = WARN( not (wrap_dmds `lengthIs` arity), ppr fn_id <+> (ppr arity $$ ppr wrap_dmds $$ ppr res_info) )
244 -- The arity should match the signature
245 (work_demands, wrap_fn, work_fn) <- mkWwBodies fun_ty wrap_dmds res_info one_shots
246 ; work_uniq <- getUniqueM
248 work_rhs = work_fn rhs
249 work_id = mkWorkerId work_uniq fn_id (exprType work_rhs)
250 `setInlinePragma` inline_prag
251 -- Any inline pragma (which sets when inlining is active)
252 -- on the original function is duplicated on the worker and wrapper
253 -- It *matters* that the pragma stays on the wrapper
254 -- It seems sensible to have it on the worker too, although we
255 -- can't think of a compelling reason. (In ptic, INLINE things are
257 `setIdNewStrictness` StrictSig (mkTopDmdType work_demands work_res_info)
258 -- Even though we may not be at top level,
259 -- it's ok to give it an empty DmdEnv
260 `setIdArity` (exprArity work_rhs)
261 -- Set the arity so that the Core Lint check that the
262 -- arity is consistent with the demand type goes through
264 wrap_rhs = wrap_fn work_id
265 wrap_id = fn_id `setIdWorkerInfo` HasWorker work_id arity
267 ; return ([(work_id, work_rhs), (wrap_id, wrap_rhs)]) })
268 -- Worker first, because wrapper mentions it
269 -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it
271 fun_ty = idType fn_id
273 arity = arityInfo fn_info -- The arity is set by the simplifier using exprEtaExpandArity
274 -- So it may be more than the number of top-level-visible lambdas
276 work_res_info | isBotRes res_info = BotRes -- Cpr stuff done by wrapper
279 one_shots = get_one_shots rhs
281 -- If the original function has one-shot arguments, it is important to
282 -- make the wrapper and worker have corresponding one-shot arguments too.
283 -- Otherwise we spuriously float stuff out of case-expression join points,
284 -- which is very annoying.
285 get_one_shots :: Expr Var -> [Bool]
286 get_one_shots (Lam b e)
287 | isId b = isOneShotLambda b : get_one_shots e
288 | otherwise = get_one_shots e
289 get_one_shots (Note _ e) = get_one_shots e
290 get_one_shots _ = noOneShotInfo
295 Suppose x is used strictly (never mind whether it has the CPR
302 splitThunk transforms like this:
305 x* = case x-rhs of { I# a -> I# a }
308 Now simplifier will transform to
311 I# a -> let x* = I# a
314 which is what we want. Now suppose x-rhs is itself a case:
316 x-rhs = case e of { T -> I# a; F -> I# b }
318 The join point will abstract over a, rather than over (which is
319 what would have happened before) which is fine.
321 Notice that x certainly has the CPR property now!
323 In fact, splitThunk uses the function argument w/w splitting
324 function, so that if x's demand is deeper (say U(U(L,L),L))
325 then the splitting will go deeper too.
328 -- splitThunk converts the *non-recursive* binding
333 -- I# y -> let x = I# y in x }
334 -- See comments above. Is it not beautifully short?
336 splitThunk :: Var -> Expr Var -> UniqSM [(Var, Expr Var)]
337 splitThunk fn_id rhs = do
338 (_, wrap_fn, work_fn) <- mkWWstr [fn_id]
339 return [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
343 %************************************************************************
345 \subsection{Functions over Demands}
347 %************************************************************************
350 worthSplittingFun :: [Demand] -> DmdResult -> Bool
351 -- True <=> the wrapper would not be an identity function
352 worthSplittingFun ds res
353 = any worth_it ds || returnsCPR res
354 -- worthSplitting returns False for an empty list of demands,
355 -- and hence do_strict_ww is False if arity is zero and there is no CPR
356 -- See Note [Worker-wrapper for bottoming functions]
358 worth_it Abs = True -- Absent arg
359 worth_it (Eval (Prod _)) = True -- Product arg to evaluate
362 worthSplittingThunk :: Maybe Demand -- Demand on the thunk
363 -> DmdResult -- CPR info for the thunk
365 worthSplittingThunk maybe_dmd res
366 = worth_it maybe_dmd || returnsCPR res
368 -- Split if the thing is unpacked
369 worth_it (Just (Eval (Prod ds))) = not (all isAbsent ds)
373 Note [Worker-wrapper for bottoming functions]
374 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
375 We used not to split if the result is bottom.
376 [Justification: there's no efficiency to be gained.]
378 But it's sometimes bad not to make a wrapper. Consider
379 fw = \x# -> let x = I# x# in case e of
383 The re-boxing code won't go away unless error_fn gets a wrapper too.
384 [We don't do reboxing now, but in general it's better to pass an
385 unboxed thing to f, and have it reboxed in the error cases....]
388 %************************************************************************
390 \subsection{The worker wrapper core}
392 %************************************************************************
394 @mkWrapper@ is called when importing a function. We have the type of
395 the function and the name of its worker, and we want to make its body (the wrapper).
398 mkWrapper :: Type -- Wrapper type
399 -> StrictSig -- Wrapper strictness info
400 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
402 mkWrapper fun_ty (StrictSig (DmdType _ demands res_info)) = do
403 (_, wrap_fn, _) <- mkWwBodies fun_ty demands res_info noOneShotInfo
406 noOneShotInfo :: [Bool]
407 noOneShotInfo = repeat False